The present invention relates in general to dimming gas discharge lamps and, more particularly, to electronic dimming ballasts for dimming compact fluorescent lamps.
A gas discharge lamp converts electrical energy into visible light with high efficiency. A gas discharge lamp is generally an elongated gas-filled (usually low-pressure mercury vapor) tube having electrodes at each end. Each electrode is typically formed from a resistive filament (usually tungsten) coated with a thermionically emissive material, such as a mixture of alkaline earth oxides.
The steady-state operation of a typical gas discharge lamp is as follows. Voltage is applied across the resistive filaments, heating the electrodes to a temperature sufficient to cause thermionic emission of electrons into the discharge tube. A voltage applied between the electrodes accelerates the electrons toward the anode. En route to the anode, the electrons collide with gas atoms to produce positive ions and additional electrons, forming in the tube a gas plasma of positive and negative charge carriers. The electrons continue to stream toward the anode and the positive ions toward the cathode, sustaining an electric discharge in the tube and further heating the electrodes. If the applied power is AC, the electrodes reverse polarity each half cycle.
The discharge causes the emission of radiation having a wavelength dependent upon the particular fill gas and the electrical parameters of the discharge. Because each collision produces additional electrons and ions, increases in the arc current cause the impedance of the lamp to decrease, a characteristic known as xe2x80x9cnegative incremental impedance.xe2x80x9d Operation of the lamp is inherently unstable, due to this negative incremental impedance characteristic, and thus the current between the electrodes must be controlled to provide stable operation of the lamp.
Gas discharge lamps, including fluorescent lamps, are designed to deliver their full rated, or xe2x80x9cnominalxe2x80x9d, light output at a specified RMS lamp current value. In this specification and the attached claims, the full rated light output of a lamp will be referred to as its xe2x80x9cnominal light outputxe2x80x9d.
Fluorescent gas discharge lamps include a phosphor coating on the inside surface of the tubular glass housing, and the excitation of this coating by radiation from the discharge provides the visible light output. Conventional fluorescent lamps are generally straight elongated tubes of essentially circular cross section with varying outside diameters ranging between about five-eighths and one and one-half inches.
Compact fluorescent lamps differ from conventional fluorescent lamps in that they are constructed of smaller diameter tubing, typically having an outside diameter of less than about five-eighths of an inch. Also, the lamps are compact in part because the tubing has one or more small radius bends that allow the tube to fold back on itself in such a manner as to achieve a compact shape. Additionally, in compact fluorescent lamps wherein the tube is folded back on itself, the lamp ends typically are in close proximity to each other.
With reference to FIG. 1, a prior art lamp system 10 includes a source of AC power such as from a 120 volt 60 Hz. sinusoidal line voltage 100, a phase controlled dimmer 102, an electronic dimmable fluorescent ballast 200, and a compact fluorescent lamp 300.
The ballast 200 receives input power (or hot) on line 202, a variable input dimming signal (or dimmed hot) on line 204, and neutral on line 206. It is understood that the voltage on line 202 is rectified by a full wave bridge rectifier 209 within the ballast 200 to yield a voltage having a positive DC average value with respect to circuit common.
The electronic dimming ballast 200 is designed to provide an amount of output power to the lamp 300 in accordance with the variable input signal on line 204 from the dimmer 102. The dimmer 102 is a phase control dimmer which provides the variable input signal on line 204 by varying its phase firing angle which controls the RMS value of the variable input signal on line 204.
As is known in the art, the ballast 200 typically includes a first power stage comprising a boost circuit 210 which receives a rectified voltage from rectifier 209 and produces a high DC voltage on line 214 which may reach 400 VDC or more.
The ballast 200 also typically includes a second power stage comprising an inverter circuit 216 which converts the DC voltage on line 214 into a high frequency switched voltage which is applied to a resonant tank circuit 230 which provides suitable AC voltage to drive the lamp 300. A high voltage energy storage capacitor 212 is provided in a shunt configuration with respect to line 214 to provide a low impedance source of current to the inverter circuit 216.
A control circuit 220 provides control signals to the boost circuit 210 and inverter circuit 216 over lines 221 and 222, respectively. The control circuit 220 controls the boost circuit 210 to provide a desired DC bus voltage and controls the inverter circuit 216 to provide the high frequency switched voltage to the resonant tank circuit 230. As a result, the ballast provides the desired current and voltage over line 208 to the lamp 300 responsive to the variable voltage input signal on line 204 such that the lamp 300 is illuminated at the proper intensity.
The control circuit 220 typically controls the inverter 216, for example, by comparing a rectified version of the variable input signal on line 204 with a signal representative of the current delivered to the lamp over line 208 and (via known error signal techniques) adjusting the control signals input to the inverter 216 over line 222 to command the proper current to the lamp 300.
As is known in the art, the control circuit 220 also commands the boost circuit 210 to produce the proper DC output voltage on line 214. Further, the control circuit 220 typically includes circuits which perform other functions such as low voltage lockout, over-current protection, over-voltage protection and the like.
In the embodiment shown in FIG. 1, power is provided by a control circuit power supply 240 to drive the control circuit 220, boost circuit 210, and inverter circuit 216. It is understood that the control circuit power supply 240 may be implemented using many circuit configurations.
The lamp system 10 of FIG. 1 requires three wires between the dimmer 102 and the ballast 200, and the ballast 200 may be located in the light fixture itself. Systems have been developed which eliminate the need for a third terminal on the ballast 200 for receiving the variable input signal on line 204. In these systems, the variable input signal is received on line 202. Other systems have been developed which utilize a third and a fourth terminal on the ballast 200 for receiving the variable input signal.
Ordinarily, when dimming linear fluorescent lamps down to low levels of light output (e.g., about one percent light output level), it is necessary to increase the output impedance of the electronic dimming ballast to maintain stable lamp operation and prevent visible flicker. Typically, the ballast output impedance is increased by driving the frequency of operation of the ballast close to the unloaded resonant frequency of the resonant tank circuit. The need for, and an apparatus and method for, obtaining high ballast output impedance is taught in U.S. Pat. No. B1 5,041,763, the entirety of which is hereby incorporated by reference.
In addition, the inventors have discovered that compact fluorescent lamps, in comparison to typical linear fluorescent lamps, have an additional area of lamp instability at low levels of lamp current around one percent of nominal light output. This additional region of instability manifests itself as a propensity for the lamp light output to extinguish, or xe2x80x9cdrop outxe2x80x9d, as opposed to flickering between various low light levels as observed in linear fluorescent lamps. While this phenomena is not fully understood, it is believed that it is related to the physical characteristics of the compact fluorescent lamp, such as the small lamp tube diameter, and the number of and small radii of the lamp bends.
Accordingly, there is a need in the art for a ballast circuit that is capable of maintaining a stable, flicker-free dimming range below about one percent of full light output for compact fluorescent lamps.
To overcome the drawbacks of the prior art ballast circuits, the present invention is directed to systems and methods for dimming a compact fluorescent lamp comprising a ballast of the type including an inverter circuit having a frequency of operation driving a resonant output tank circuit having a predetermined unloaded resonant frequency. The frequency of operation of the inverter circuit is selected such that the open loop system gain is below a first predetermined level (e.g., below about 15) and the ballast output impedance is above a second predetermined level (e.g., above about twice the absolute value of the maximum negative incremental lamp impedance).
According to aspects of the invention, the frequency of operation of the inverter circuit is determined by a control circuit comprising a frequency-determining resistor-capacitor (RC) network. The component values of the RC network are chosen such that the frequency of operation of the inverter circuit, at a lamp output of less than about one percent of nominal light output, is a predetermined function of the resonant frequency of the unloaded resonant tank circuit.
For purposes of this specification and the appended claims, the term xe2x80x9cDCxe2x80x9d refers to a voltage or current waveform that is unidirectional and can be either pulsating or non-pulsating. The term xe2x80x9cACxe2x80x9d refers to a voltage or current waveform that reverses polarity at regularly recurring intervals of time and has alternately positive and negative values. The term xe2x80x9cDC componentxe2x80x9d refers to the average value of an AC or DC waveform. The term xe2x80x9cAC componentxe2x80x9d refers to that portion of an AC or DC waveform remaining after its DC component has been subtracted.